(1) Field of the Invention
The invention relates to a voltage controlled variable capacitor, and more particularly, to a variable capacitor, formed of a larger number of fixed capacitor segments and a corresponding number of switching elements, which are typically integrated with the capacitance controlling functions on an integrated semiconductor circuit.
(2) Description of Prior Art
One example of a voltage-controlled capacitor is a varactor diode. When a reverse voltage is applied to a PN junction, it creates a depletion region, essentially devoid of carriers, which behaves as the dielectric of a capacitor. The depletion region increases as reverse voltage across it increases; thus the junction capacitance will decrease as the voltage across the PN junction increases. However the characteristics are non-linear and are widely temperature and process dependent. There is also a significant leakage current problem. Varactor diodes must be operated below the junction breakdown voltage. The varactor diode is sometimes called a varicap.
FIG. 1a shows the principle of a varactor diode; FIG. 1b shows the control voltage to capacitance characteristics of said varactor diode and demonstrates the effects of temperature and process variations. Another example is a switched capacitor chain, where capacitors are switched in parallel one after the other, thus increasing the capacitance step by step. The capacitors, when made of metal or polycarbonate structures, are far less sensitive to temperature and process deviations.
FIG. 2a shows the basic circuit concept. However, as is demonstrated in FIG. 2b, there is only a “step-wise linear” capacitance change over the control voltage. In addition the switching of the individual capacitors causes switching noise (“spikes”) on the common circuit rails. Furthermore, while the switching transistor is kept in flat switching ramp to smooth the switching steps, the transistor's resistance causes a Q-factor problem.
U.S. Pat. No. 6,356,135 (to Rastegar) describes an electronically trimable capacitor having a plurality of branch circuits, each including a capacitor which may be selectively controlled by a switch to contribute or not to the net capacitance exhibited by the trimable capacitor. Operation of the switches is under direction of digital instruction.
U.S. Pat. No. 5,514,999 (to Koifman, et al.) shows a differential switched capacitor circuit, comprising: multiple switched capacitor stages, coupled in a chain.
U.S. Pat. Nos. 4,449,141 and 4,456,917 (to Sato, et al.) disclose a variable capacitor comprising a plurality of variable capacitor elements each having depletion layer control sections and a capacity reading section formed on a semiconductor substrate so that the capacity appearing at each capacity reading section varies in response to the bias voltage applied to the depletion layer control sections.